1. Field of the Invention
The present invention relates to a clock regeneration apparatus, and more particularly to a clock regeneration apparatus disposed in each transmission unit for regenerating and distributing a timing signal for a data transmission network that is organized by a plurality of transmission units linked in a ring topology, together with external clock sources locally connected to at least two transmission units.
2. Description of the Related Art
In a synchronous network interconnecting a plurality of transmission units (or nodes) in a ring topology, it is required for all the units to operate in synchronization with a common reference clock signal. To this end, an appropriate synchronization distribution mechanism has to be incorporated in the system. Furthermore, to improve the reliability of clock timing distribution, the network often employs two or more clock signal generators connected to separate transmission units. Although a plurality of clock sources are prepared, the network system actually uses only one source at a time in normal situations, while leaving the others as backup sources. To allow the transmission units in the network to choose a correct clock source, some messages are distributed to indicate the quality status of the clock signal being used. Each transmission unit uses these messages, together with a list of available clock sources, to make an appropriate selection, so that the overall system will operate in synchronization with a common clock source.
FIG. 17 shows an example of a conventional ring network, where four transmission units 101 to 104 are linked by optical link cables in a ring topology. Note here that the network has two rings running in opposite directions. In the system of FIG. 17, two external clock sources 105 and 106 are connected to the transmission units 101 and 103, respectively, to supply clock signals having a quality level of G811.
The text strings shown in each box of the transmission units 101 to 104 indicate a "selected clock source" and a "clock source list." The "selected clock source," which is enclosed in angle brackets (e.g., &lt;EXT&gt;), shows a particular clock source that the transmission unit has selected as its timing reference for data transmission (as will be described later). The "clock source list" provides a list of potential clock sources with priorities, allowing each transmission unit to select an appropriate timing reference. More specifically, a symbol "PRI1" denotes the highest priority, while another symbol "PRI3" the lowest. Still another symbol "EXT" represents an external clock source that is local to the transmission unit. Symbols "GP1" and "GP2" are used to distinguish between two optical link cables extending from each transmission unit to its adjacent transmission units. When viewed from a transmission unit, the optical link cable on one side is called "GP1 line," while the same on the other side is called "GP2 line." As an alternative interpretation, these "GP1" and "GP2" can be understood as the labels of interface ports of a transmission unit, rather than those of optical link cables. In FIG. 17, symbols "G811" and "DUS" are placed on the individual optical link cables interconnecting the transmission units 101 to 104, which represent messages indicating the quality levels of clock signals being delivered over the cables. The DUS ("Don't Use for Sync") messages inhibit the downstream transmission units from using that line clock source for synchronization.
FIG. 17 illustrates a normal situation where both external clock sources 105 and 106 are functioning correctly. The transmission unit 101 selects the external clock source 105 as the synchronization reference signal, since its internal clock source list gives the highest priority to an external clock source (EXT) local to the unit. Other transmission units 102, 103, and 104 select line clock sources provided from their respective GP1 lines, according to their own clock source lists. As a result, all the transmission units 101 to 104 are timed by the external clock source 105.
Referring now to FIG. 18, the following section will present how the network of FIG. 17 behaves when the external clock source 105 has failed. FIG. 18 shows the synchronization distribution path over which the clock signals propagate in such a fault situation. Now that the external clock source 105 is inoperative, the transmission unit 103 selects the other external clock source 106 as the alternative source for synchronization reference. The transmission units 101 and 104 select their respective GP2 lines, and the transmission unit 102 its GP1 line, in accordance with their individual clock source lists. Finally, a new synchronization distribution path is established as indicated by the bold arrows in FIG. 18, where every transmission unit is timed by the new external clock source 106. In this way, the network gains an improved reliability of timing synchronization by employing a plurality of external clock sources.
The above-described conventional network, however, has such a drawback that the consistency in the synchronization timing would be lost when some problem occurred in a link between two transmission units. That is, it can potentially happen that some transmission units synchronize themselves with one external clock source, while other units synchronize themselves with another source. More specifically, FIG. 19 shows a specific fault situation where the link between the transmission units 101 and 104 is disrupted, while both external clocks have no problems. Being unable to receive clock signals from the transmission unit 101 via the transmission unit 104, the transmission unit 103 selects the external clock source 106 according to its own clock source list. Consequently, the transmission units 102 and 104 become synchronized with the external clock source 106. The transmission unit 101, however, still selects its external clock source 105, because the entry "EXT" has the highest selection priority. In this way, a link failure can cause an undesired condition where the transmission units within a network are synchronized not with a single common clock source, but with two different sources.
FIG. 20 shows still another fault situation where the external clock source 105 is inoperative, and the link between the transmission units 103 and 104 has encountered a problem. Since the external clock source 105 is lost, the network attempts to reconfigure itself to use the external clock source 106 for synchronization distribution as shown in FIG. 18. However, a subsequent link failure between the transmission units 103 and 104 disables the provision of a clock signal from the transmission unit 103 to the next transmission unit 104. Now that the transmission unit 104 has no clock sources to trace, it enters another timing mode in which its internal clock signal is locked as is. This is known as a "holdover" mode, and the outgoing clock signal in this mode (i.e., "holdover" clock) is represented by a symbol "HO" in FIG. 20. The transmission unit 101 then selects the line clock source GP2 for synchronization, in accordance with its own clock source list. In this way, the network falls into an inconsistent situation in term of the clock source selection. That is, the transmission units within a network operate in synchronization not with a single common clock source, but with two separate sources.
The above undesired situation may be avoided by making the clock source list of the transmission unit 101 include both entries of "GP1" and "GP2." This setup, however, has a side effect described as follows.
FIG. 21 illustrates a network configuration where both "GP1" and "GP2" are registered in the clock source list of the transmission unit 101. More specifically, the clock source list has three entries "EXT," "GP1," and "GP2" with successively lower priorities. This setup allows all the transmission units to be timed by the external clock source 105, as long as the source 105 operates correctly. However, a problem with the external clock source 105 would lead to a closed loop of synchronization distribution, as shown in FIG. 21, where all the transmission units selects their respective "GP1" line clocks. This explains why the transmission unit 101 is allowed to contain not both of, but only one of the GP1 and GP2 line timing sources in its clock source list.